High contact count, sub-miniature, fully implantable cochlear prosthesis

ABSTRACT

A fully implantable cochlear prosthesis includes ( 1 ) an implantable hermetically sealed case wherein electronic circuitry, including a battery and an implantable microphone, are housed, ( 2 ) an active electrode array that provides a programmable number of electrode contacts through which stimulation current may be selectively delivered to surrounding tissue, preferably through the use of appropriate stimulation groups, and ( 3 ) a connector that allows the active electrode array to be detachably connected with the electronic circuitry within the sealed case. The active electrode array provides a large number of both medial and lateral contacts, any one of which may be selected to apply a stimulus pulse through active switching elements included within the array. The active switching elements included within the array operate at a very low compliance voltage, thereby reducing power consumption. The entire prosthesis is very efficient from a power consumption standpoint, thereby allowing a smaller battery to power the system for longer periods of time before recharging or replacement is required. The hermetically sealed case within which the electronic circuitry, battery, and microphone are housed may be replaced, when needed, through minimally invasive surgery. Further, the electronic circuitry housed within the hermetically sealed case may be programmed, as needed, using acoustic and/or RF control signals. In one embodiment, such control signals may be realized using phase-shift keyed (PSK) modulation of an acoustic signal within a very narrow frequency band centered at about 6 KHz.

[0001] The present application is a Divisional of U.S. application Ser.No. 09/823,271, filed Mar. 30, 2001; which claims the benefit of U.S.Provisional Application Serial No. 60/193,647, filed Mar. 31, 2000; andU.S. Provisional Application Serial No. 60/203,707, filed May 11, 2000,which applications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to implantable devices, and moreparticularly, to a fully implantable device or system for stimulating orsensing living tissue. The implantable device includes a rechargeablebattery or other replenishable power source. More particularly, thepresent invention relates to a fully implantable cochlear implant system(FICIS) that allows profoundly deaf persons to hear sounds without theneed for wearing or carrying external (non-implanted) hearing devices orcomponents.

[0003] Presently available implantable stimulation devices, such as acochlear implant device or a neural stimulator, typically have animplanted unit, an external ac coil, and an external control unit andpower source. The external control unit and power source includes asuitable control processor and other circuitry that generates and sendsthe appropriate command and power signals to the implanted unit toenable it to carry out its intended function. The external control unitand power source are powered by a battery that supplies electrical powerthrough the ac coil to the implanted unit via inductive coupling forproviding power for any necessary signal processing and controlcircuitry and for electrically stimulating select nerves or muscles.Efficient power transmission through a patient's skin from the externalunit to the implanted unit via inductive coupling requires constantclose alignment between the two units.

[0004] Representative prior art cochlear implant systems are disclosed,e.g., in U.S. Pat. Nos. 4,532,930; 4,592,359; 4,947,844; 5,776,172; and6,067,474, all of which are incorporated herein by reference.

[0005] Disadvantageously, each of the known prior art cochlearstimulation systems, with the exception of some embodiments of thesystem disclosed in the U.S. Pat. No. 6,067,474 patent, requires the useof an external power source and speech processing system, coupled to theimplanted stimulation device. For many patients, achieving andmaintaining the required coupling between the external components andthe implanted component can be troublesome, inconvenient, and unsightly.Thus, there exists a need and desire for a small, lightweight fullyimplantable device or system that does not require an external unit inorder to be fully functional, that does not need constant externalpower, and that includes a long-lasting internal battery that may berecharged, when necessary, within a relatively short time period.

[0006] Moreover, even if a rechargeable battery were available for usewithin an implantable cochlear stimulation system, such rechargeablebattery must not significantly alter the size of the existingimplantable cochlear stimulator. This is because the curvature andthickness of the skull is such that there is only a limited amount ofspace wherein a surgeon may form a pocket wherein a cochlear stimulatormay be implanted. This is particularly an acute problem for youngchildren, where the thickness of the skull is relatively thin and thecurvature of the skull is greater than for an adult. Thus, there is aneed for a fully implantable cochlear implant system that is adaptableand lends itself for implantation within a range of head sizes andshapes.

[0007] Additionally, even where a rechargeable battery is employedwithin a fully implantable cochlear implant system, which fullyimplantable system includes an implantable speech processor andmicrophone, it may be necessary or desirable, from time to time, toreplace the battery and/or to upgrade the speech processor hardware.Because implantation of the cochlear implant system, including insertionof the delicate electrode array into the cochlea of the patient,represents major surgery, which major surgery would hopefully only needto be performed once in a patient's lifetime, it is seen that there isalso a need for a fully implantable cochlear implant system wherein atleast the battery, and perhaps even some or all of the speech processingcircuitry, may be replaced or upgraded from time to time through minimalinvasive surgery, while leaving the delicate cochlear electrode arrayintact for use with the replaced battery and/or upgraded speechprocessor.

BRIEF SUMMARY OF THE INVENTION

[0008] The present invention addresses the above and other needs byproviding a fully implantable cochlear prosthesis that includes (1) animplantable hermetically sealed case wherein electronic circuitry,including an implantable microphone, are housed, (2) an active electrodearray that provides a programmable number of electrode contacts throughwhich stimulation current may be selectively delivered to surroundingtissue, and (3) a connector that allows the active electrode array to bedetachably connected with the electronic circuitry within the sealedcase.

[0009] In accordance with one aspect of the invention, the activeelectrode array provides a plurality of groups of electrodes, eachelectrode having both medial and lateral contacts, any one of which maybe selected to apply a stimulus pulse through active switching elementsincluded within the array, preferably through the use of StimulationGroups.

[0010] In accordance with another aspect of the invention, the activeswitching elements included within the array operate at a very lowcompliance voltage, thereby reducing power consumption.

[0011] In accordance with still another aspect of the invention, radialstimulation may be provided by the active electrode array in order toincrease selectivity.

[0012] In accordance with yet another aspect of the invention, theentire prosthesis is very efficient from a power consumption standpoint,thereby allowing a smaller battery to power the system for longerperiods of time before recharging or replacement is required.

[0013] In accordance with a further aspect of the invention, thehermetically sealed case within which the electronic circuitry, battery,and microphone are housed may be replaced, when needed, throughminimally invasive surgery.

[0014] In accordance with an additional aspect of the invention, theelectronic circuitry housed within the hermetically sealed case may beprogrammed or adjusted, e.g., upgraded, as needed, using either RF oracoustic control signals received through an implantable coil or animplantable microphone. In a preferred embodiment, acoustic controlsignals are realized using phase-shift keyed (PSK) modulation of anacoustic signal within a very narrow band centered at about 6 KHz.

[0015] It is thus an object of the present invention to provide a fullyimplantable tissue stimulation prosthesis that utilizes an activeelectrode array.

[0016] It is another object of the invention to provide such a fullyimplantable prosthesis that includes both digital and analog circuits,any one or all of which may be used depending upon the selected mode ofoperation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The above and other aspects, features and advantages of thepresent invention will be more apparent from the following moreparticular description thereof, presented in conjunction with thefollowing drawings wherein:

[0018]FIG. 1 is a functional block diagram of a fully implantablecochlear prosthesis made in accordance with the present invention (whichfigure is split into two halves, one being denoted FIGS. 1-1 and theother FIGS. 1-2);

[0019]FIG. 2 is a diagram that illustrates the physical arrangement ofthe main elements of the invention;

[0020]FIG. 3 illustrates the medial and lateral contacts and electricalcircuitry carried on and within the active electrode array that is usedwith the invention;

[0021]FIG. 4 depicts the manner in which the electrode contacts arefabricated using active cells;

[0022]FIG. 5 illustrates the manner in which the active cells arestacked together in order to form the active electrode array;

[0023]FIG. 6 is a block diagram that illustrates a hardware-basedcurrent balancing approach that may be used by the invention;

[0024]FIG. 6A is a waveform diagram associated with thecurrent-balancing approach illustrated in FIG. 6; and

[0025]FIG. 7 is a timing waveform diagram that illustrates thestimulation group concept utilized by the invention.

[0026] Corresponding reference characters indicate correspondingcomponents throughout the several views of the drawings.

DETAILED DESCRIPTION OF THE INVENTION

[0027] The following description is of the best mode presentlycontemplated for carrying out the invention. This description is not tobe taken in a limiting sense, but is made merely for the purpose ofdescribing the general principles of the invention. The scope of theinvention should be determined with reference to the claims.

[0028] While the invention described herein is presented in terms of acochlear prosthesis, it is to be understood that a cochlear prosthesisis only one example of numerous different types of prosthesis andapplications that may benefit from the invention. That is, the cochlearprosthesis described herein is, in reality, a neural stimulator, andsuch neural stimulator may also be used for applications other thanstimulation of the cochlea. For example, the prosthesis described hereinmay be used to selectively stimulate any tissue or nerves throughout apatient's body for a wide variety of applications, including deep brainstimulation, stimulation to control urinary incontinence, stimulation tomanage pain, stimulation to control various nervous disorders anddiseases, and the like. Further, it is to be understood that the activeelectrode described herein, while intended for insertion into a humancochlea, may also be used for many other types of applications. All suchother types of applications are considered to fall within the scope ofthe present invention.

[0029] The following patents or pending patent applications discloseinformation relevant to the design, use and operation of a fullyimplantable cochlear stimulation system: U.S. Pat. No. 5,603,726; U.S.Pat. No. 6,002,966; U.S. patent application Ser. No. 09/407,826, filedNov. 28, 1999; U.S. patent application Ser. No. 09/322,712; whichpatents and patent applications are incorporated herein by reference.

[0030] In order to provide a miniature, low power, cochlear prosthesis,a totally new design approach is utilized. Such design approach is,unfortunately, not backward compatible with existing cochlear implantdevices and cochlear electrode arrays. However, as will be evident fromthe description that follows, the benefits provided by the miniature,low power prosthesis of the present invention far outweigh thedisadvantages for those patients who need such a prosthesis.

[0031] The features and advantages of the cochlear prosthesis describedherein may be summarized as follows:

[0032] (1) Fully Simultaneous Stimulation and analog processing is notsupported (four simultaneous channels maximum), thereby greatlysimplifying the electronic circuitry needed within the prosthesis.

[0033] (2) Back compatibility with previous devices is not supported,again simplifying the circuitry needed.

[0034] (3) Support for Radial stimulation is provided, thereby offeringincreased selectivity.

[0035] (4) Low Compliance Electrodes are provided that reduce power.

[0036] (5) The “Lock” requirement is removed which allows the system tooperate asynchronously, which also greatly simplifies the circuitry.

[0037] (6) A connector on the electrode with active electronics in eachcontact is provided. This simplifies the electronic circuitry that mustinterface with the electrode contacts, and allows the electrode array tobe detached from the electronic housing, if needed (e.g., to replace adefective or depleted battery).

[0038] (7) The need to use a capacitor to provide capacitive couplingfor each contact is eliminated. The function provided by the couplingcapacitor is advantageously replaced by a current servo, as explainedbelow.

[0039] In short, the above features provided by the inventiondramatically reduce power consumption and system size. The design allowsimplementation with either analog or digital processing. The removal ofthe back-compatibility requirement greatly reduces the systemcomplexity.

[0040] It is also to be noted that while the following description iswritten in the context of a one dimensional electrode array, it caneasily be applied to a multidimensional array(s) of electrodes.Unidimensional and multidimensional electrode arrays, as taught by thepresent invention, may be applied to a multiplicity of neuralstimulation systems such as cochlear, retinal, brain, spinal and organsystems.

[0041] Turning first to FIG. 2, the components of the present inventionare illustrated. A hermetically sealed housing 100 is adapted to beimplanted under the skin 102 of a patient. The housing 100 includeselectronic circuitry (not shown in FIG. 2, but functionally depicted inFIG. 1) coupled to an active electrode array 29 through a connector 9.The active electrode array 29 includes a plurality of groups ofmedial/lateral pairs of electrodes. In a preferred embodiment, theconnector 9 has five feed-through connectors 15 through which fivedifferent signal conductors included within the active electrode array29 are connected to the circuitry within the housing 100. Morefeed-through connectors 15 may be used, as required. The five signalconductors included within the active electrode array 29 are connectedto four groups of four medial/lateral pairs of electrodes, as explainedbelow. A subcutaneous microphone 1 is attached to the main body of thehousing 100. (Other types of subcutaneous microphones orin-the-ear-canal microphones may be used in lieu of, or in addition to,the microphone 1 attached to the housing 100.) An RF coil 30 is alsoattached to the housing, and is electrically connected to the circuitrywithin the housing.

[0042] An external (non-implanted) charging/programming unit 104 sendsand receives RF signals to and from the RF coil 30, and hence to andfrom the implanted circuitry within the housing 100. Such signals areused to charge a battery within the housing 100 and/or to program thecircuitry within the housing, as is known in the art. Acoustic controlsignals may also be sent to the implantable microphone 1 from anexternal acoustic remote control 106. Such acoustic control signals aretypically used by the patient to alter the operation of the cochlearprosthesis within prescribed limits, e.g., to adjust the volume orsensitivity.

[0043] Next, turning to FIG. 1, the operation of the prosthesis providedby the present invention will be explained. Acoustic energy between 100and 6000 Hz enters transcutaneously through a subcutaneous microphone 1in the main body of the implant (and/or through other types ofmicrophones, not shown). This acoustic energy consists of sound to beprocessed and sent to the patient, as well as narrow bandphase-shift-keyed commands (6 KHz) for device control and programming.The microphone signal is amplified by amplifier 2 approximately 20 dBand sent to either analog or digital front end circuitry forpreprocessing. A Remote PSK demodulator 3 always searches for PSK data.Demodulated command data is sent to a digital Microprocessor ControlUnit (MCU) 4 for decoding. The presence of the command data will wake upthe MCU 4 if it is asleep. The MCU 4 can be commanded into various modesof operation by these commands. Such modes include at least: (1) aserial boot mode; (2) a battery maintenance mode; (3) a fitting mode;(4) a sleep mode; and (5) a normal operating mode with a remote control.

[0044] Still with reference to FIG. 1, after pre-amplification ordigitization, the first step in the processing chain is a bulk automaticgain control circuit 5. This automatic gain control brings the signalinto the dynamic range of an Acoustic Processing Bank 6. The AcousticProcessing Bank 6 includes 1-64 configurable filters and envelopedetectors, or equivalent taps for envelope information. Typically, acontact is provided for each tap. If there are more electrodes thantaps, then variable sized contacts or optimal selections of electrodescan be exploited. Having more taps than contacts (a concept known as“virtual electrodes”) is not supported in this architecture because itwould require more and scalable current sources. Note that virtualelectrodes require a mechanism to send single acoustic tap informationto a multiplicity of electrode array contacts that are “scaled” to theposition in the array or the weighting of the contact. (In arepresentative cochlear stimulation system, for example, disclosed inU.S. Pat. No. 6,002,966, incorporated herein by reference, this is doneby storing weights for each temporal-spatial stimulus nexus.)Additionally, even though virtual electrodes have shown to be viable forincreasing the spatial/pitch resolution for simple signals, theirviability for complex acoustic signals due to interaction effects hasnot been well established. The present invention, on the other hand,supports very sophisticated Acoustic Processing Banks, such as theSilicon Cochlea with Distributed Adaptive Gain, see Sarpeshkar, et al.,“A Low-Power Wide-Dynamic-Range Analog VLSI Cochlea”. Analog IntegratedCircuits and Signal Processing, 16, 245-274 (1998), incorporated hereinby reference.

[0045] A four output cross-point switch 7 allows the selection of anyfour of the available taps to be routed to any output. A fullcross-point switch is preferred for the purposes of adjusting to patientpathology, correcting electrode position, and allowing drivers to beinterspersed along the array to reconfigure for partial device failuresand produce arbitrary longitudinal bipolar drive configurations. Fouroutputs are selected because of the ability to implement a practicalelectrode connector with five contacts. Such a contact may besubstantially as described in U.S. patent application Ser. No.09/455,046, filed Dec. 6, 1999, incorporated herein by reference.Additionally, with only five contacts, the number of wires is reducedover what has previously been required, and this also allows the wiregauge to be increased, all of which has reliability and surgicalimplications. Additionally, while some might be concerned that havingonly four simultaneous stimulation sites is a limitation, it is believedthat four simultaneous stimulation sites may in reality be close to themaximum number of channels that can be used while still avoidingexcessive interaction. Nonetheless, as the connector and electrodetechnology evolves, the number of simultaneous electrodes may beincreased, if such proves to be beneficial. The major disadvantage ofproviding only four simultaneous drives is the resulting limitation ononly being able to drive two true longitudinal bipolar channelssimultaneously. Such may or may not be an issue depending upon whetherradial stimulation modes are more efficacious for a majority ofpatients.

[0046] After the cross-point switch 7, a Log circuit 8 takes thelogarithm of the signal. The Log function approximates a patient'sperception of acoustic loudness as a function of current. While the Logfunction does not represent a perfect predictor of the patient's orsubject's perception of acoustic loudness, it is one of the bestpredictors that has been found that is clinically efficacious. This isdue, in large part, to the patients' ability to hear through loudnessvariations and various forms of compression.

[0047] The output of the Log circuits 8 are summed in summing circuit12. The output of summing circuit 12 is compared with relevant digitaldata obtained from the MCU 4, which relevant digital data is firstconverted back to analog data with a digital-to-analog converter (DAC)circuit prior to the comparison.

[0048] Next, still with reference to FIG. 1, the log of the acousticsignal is mapped into the patient's electrical dynamic range by a Mappercircuit 9A. The Mapper circuit 9A modifies the signal by two constants(A*input+K) that are independently derived for each channel based uponthe behavioral threshold and most comfortable level of each stimulationsite. The constants A and K are also dependent upon the patient's orsubject's volume control and input dynamic range control, and can bemodified up to 10 times per second.

[0049] The output of each Mapper circuit drives a Current Source 10. TheCurrent Source 10 is coupled through an optional electrical failureprotection capacitor 14 to the Active Contacts 16 in the Electrode Array29. The Current Sources 10 are usually open (and thus represent aninfinite impedance) and are controlled by Pulse Generators 13, andultimately by the MCU 4. The Current Sources 10 operate in one of thefollowing modes: (1) positive (+) current; (2) negative (−) current; (3)open; (4) short to ground; and (5) 500 kilo-ohm leak to ground. Currentsource noise is controlled such that 64 monotonic resolvable steps areavailable to the patient with a current range of 3-30 dB (referenced to1 μA) over a maximum current range of 20 to 2000 μA. The slew rate ofthe output currents from the Current Sources 10 is between about1-10V/μS with an assumed circuit of a series 1.0 μH inductor and a loadof 10 to 100 nanofarads with an impedance in the range of 500-20,000ohms. The current sources 10 obtain their operating power from either abattery or a fast tracking Buck and/or Boost converter 11, depending onthe maximum compliance voltage required by electrode technology andstimulation strategy. A fast tracking converter, if used, should havethe ability to track the speech envelope to maximize efficiency. Usingmultiple Buck/Boost converters does not offer a significant advantagebecause the varying compliance voltage requirements throughout thecochlea are minimized through the use of variable pulse-width capabilityand variable contact size. Representative maximum compliance voltagesrequired as a function of electrode configuration are:

[0050] 1. 16 Channel and Platinum Contacts (16 Volts).

[0051] 2. 16 Channel and Coatings (3 Volts).

[0052] 3. 64 Channel, Coatings and Small Contacts (9 Volts).

[0053] The Pulse Generators 13 which control the Current Sources 10receive commands from a Pulse Table in the MCU (4). The use of pulsetables is explained in the referenced patents and patent applications. Agiven Pulse Generator 13 is commanded from the MCU to select a specificacoustic tap (filter) and to turn on a set of Active Contacts 16, eachof which can enable their respective Medial and/or Lateral Contacts. ThePulse Generator 13 coordinates the detailed timing and current driverequired to keep charge in balance on individual contacts and disallowscontact switching until the charge is balanced. The Pulse Generatorscommunicate to the Active Contacts 16 over the same line that theCurrent Sources 10 use. This is done by sending digital data at afrequency much higher than the stimulus current transitions with no DCoffset as guaranteed by another small capacitor 14.

[0054] As seen in FIG. 3, the Active Contacts 16 of the active electrodearray 29 have a Current Input line 17 from a specific Current Source 10and Pulse Generator 13, and a Reference Input line 18 from anIndifferent Current Driver 21. The Current Input line 17 carries a datastream that is decoded in a Decode circuit 16A by each Active Electrode16 in order to control Bilateral Switches (BiSW) 19, 20 so as to connector disconnect a Medial Contact Plate 19A to the Current Source line 17and/or to connect a Lateral Contact Plate 20A to the Indifferent CurrentSource 21 through the Reference Input line 18. The Reference Input line18 is used to provide a ground for the decode circuitry 16A and also canbe connected to the Lateral Contact Plate 20A as mentioned above. Whenboth the Medial and Lateral Contact Plate are connected, RadialStimulation is activated, providing potentially a more selectivestimulation source. Rectification of a very small amount of power occursto provide operating power for the Decode circuit 16A and to bias theBilateral Switches 19 and 20. The current is kept very small so that ifthere is a failure of the contact, only a small amount of direct currentleakage will leave the contact. Hence, when the leakage current issufficiently small, this type of failure does not require explant.

[0055] Turning next to FIGS. 4 and 5, the fabrication of the activeelectrode array 29 is illustrated. The Active Electrodes 16 providedwithin the array 29 comprise hermetically sealed active circuitsfabricated on a single silicon-on-sapphire (aluminum oxide, AlO) die,with four platinum electrical connections that leave or exit thehermetic space—two connections for contact plates 19A and 20A and twoconnections for inputs through signal lines 17 and 18 (see FIG. 3). Thehermetic seal is preferably achieved with an aluminum oxide coating thatis placed directly over the silicon portion of the die, whereas thesapphire substrate is inert (so it does not need to be coated). Avoid-free silastic over-coating may also be used to protect the aluminumoxide from etching due to possible acidic build-ups resulting fromstimulation. Alternatively, a ceramic housing may be provided havingcontacts that are laser welded onto the ceramic to seal the device.

[0056]FIG. 4 illustrates the fabrication of one active electrode 16 ofthe active electrode array 29. Platinum (Pt) contacts 200 are welded toa silicon die 202 on which appropriate circuitry has been fabricated toperform the function of the bilateral switches 19 and 20 and the decodecircuit 16A. The contacts 200 are coated with aluminum oxide (AlO). Theoxide is etched off of the contacts 200, leaving an exposed surface area201. The exposed contacts are then bent over the edge of the siliconedie 202, in the direction of the arrow 205, thereby forming the medialand lateral contacts 19A and 20A. Platinum lobes 206, 207 are alsoattached to the silicone die 202 to allow electrical contact with thecircuitry formed thereon.

[0057]FIG. 5 shows the stacking of several of the active electrodesformed in FIG. 4 into a bank of electrodes. The wires 17 and 18 areattached, e.g., through a crimping process, to the Platinum lobes 206.The wires themselves serve as a flexible spine support for keeping theindividual sections (FIG. 4) separated from each other. Alternatively,flexible spacers may also be inserted in between the individualelectrode sections.

[0058] Once the Active Electrodes 16 are stacked together, they are alsomolded with silastic into the electrode array 29. Two indifferent wires18 and one wire 17 from the driver are connected to each ActiveElectrode. The connection is made through a splice and crimp to thelobes 206 protruding from the silicone die. Two indifferent wires 18 areused for reliability in the event that one fails. A 64-channel electrodearray is arranged such that different banks overlap every millimeter.This arrangement advantageously lends itself to several configurations:

[0059] 1. Up to 16 electronically variable size contacts (0.25, 0.50,0.75, or 1 mm) or 32 variable size contacts (0.25 or 0.50) may beemployed.

[0060] 2. Up to 2 longitudinal bipolar contacts, with spacing of 1-16 mmin 1-mm increments, at up to 32 sites may be utilized.

[0061] 3. Up to 4 radial semi-bipolar (same ground for all contacts) atup to 64 sites may be offered.

[0062] 4. Up to 4 simultaneous monopolar electrodes at up to 64 sitesmay be offered.

[0063] 5. Up to 4 simultaneous radial electrodes with common ground orrib electrodes may be provided.

[0064] 6. Up to 1 non-simultaneous true bipolar stimulation site may beoffered.

[0065] Should any one bank of active electrodes fail, one 1-mm sectionof the electrode array will be out of commission. Moreover, it is notedthat each, or at least one, active electrode in each bank may have abuilt in capacitive strain gauge incorporated therein. This gaugemeasures the stress across the die indicating the stress setup by theattachment wires and cochlear structures contacting the array orcontacts. Such sensors (strain gauges) further help surgeons perform alow stress insertion of the array, and also detect any significantproblems during insertion. The output signal(s) of the sensors aretypically scanned by having the current sources issue a sinusoid currentand then measuring the phase shift with the Back Telemetry MeasurementSystem.

[0066] Returning to FIG. 1, it is seen that the Indifferent CurrentSource 21 (also referred to as an Indifferent Current Driver) driveseither an Indifferent Case Electrode 23, or the Lateral Contacts 20A ofthe Active Electrodes when commanded. A Bilateral Switch 22 under MCUcontrol enables the Indifferent Case Electrode 23. The Pulse Table inthe MCU 4 controls both the Indifferent Current Source 21 and theBilateral Switch 22. The capability of the Indifferent Current Source isthe same as the other Current Sources 10, except than it can sink orsource up to 8 mA.

[0067] A Back Telemetry Measurement System 39 measures voltages on anyone or two sets of contacts differentially. The gain of a differentialamplifier 40 can be set to 1, 3, 10, 30, 100, 300 or 1000. The recoverytime of the amplifier 40 is less than 35 μS. A 9-bit ADC 41 samples atup to 60K samples per second and stores the results in memory circuitsincluded within the MCU 4.

[0068] An RF coil 30 receives power and may transmit back telemetrydata. The received power is rectified by diode D1 and powers a linearLithium Ion Battery Charger 33 to charge an implanted battery 34. ABattery Protection Circuit 35 protects the battery from conditions suchas over charge and over-discharge, automatically disconnecting thesource or load when necessary. The system can still operate from anexternal source through the coil 30 if the battery is disconnected. ABuck Converter Circuit(s) 36 derives the necessary power supply voltagesfrom the battery voltage that are required for operation of theprosthesis.

[0069] A Back Telemetry Transmitter function is included within the MCU4. An RF carrier with digitally encoded modulated data is generated bythe MCU 4. Such modulated RF carrier is transmitted through couplingcapacitor 31 onto back telemetry signal path 32 to antenna coil 30,where it is transmitted and ultimately received by an external system. Aprogramming adapter 104 and/or 106 (FIG. 2) has an acoustic transducerfor sending data to the prosthesis, and a Back Telemetry Receiver anddecoder for receiving data back from the prosthesis. The programmingadapter is designed such that it can operate at a distance of up to onemeter radiating through the RF coil 30 alone. An alternative mode ofoperation provides that the Back Telemetry Transmitter simply sends ashort duration high amplitude RF carrier pulse that can be received at adistance from the body by a hand-held remote unit. This providespatients or care-givers knowledge of the system status and system modesrequired of the fully implanted system.

[0070] The MCU 4 is self-clocked off an internal crystal 37, therebyadvantageously eliminating the need to obtain “lock” which hastraditionally been required by existing implantable cochlear stimulatorsystems. The internal crystal 37 also provides a clock signal from whichthe RF carrier signal (for back telemetry) may be derived. The systemthus runs entirely asynchronous relative to outside support units. Forbinaural applications, an acoustic command and back telemetry canperiodically synchronize two systems, as required, if the systems areequipped with the ability to detect the presence of an external Backtelemetry carrier. Advantageously, the MCU 4 does not need non-volatilememory since it always has battery voltage. Accordingly, the MCU is ableto perform a serial boot at the clinic with the fitting system over theacoustic data link when the battery is fully discharged.

[0071] Psychophysical testing and impedance measurements are performedusing pulses generated from a pulse table. In this mode, the input tothe current sources must be settable to a fixed value so that pulsemodulation can be performed.

[0072] A preferred use of the cochlear prosthesis of the presentinvention is with simultaneous pulsatile strategies. Simultaneouspulsatile strategies require careful coordination of the outputs inorder to avoid interaction and maximize information transfer. In orderto understand better how this basic approach operates, the concept ofStimulation Groups will next be described. Simply stated, StimulationGroups are groupings of contacts that are stimulated together, asillustrated in FIG. 7. The grouping of contacts is determined byelectrode interaction and compound action potential measures. Throughthese measures, the number of actual channels (or useful electrodes) canbe estimated by reducing, e.g., a 64×64 interaction matrix. The inverseof this matrix yields a direct measure of interaction guiding thesorting of contacts into stimulation groups. (Typically, when using thisapproach, a 64^(th) order matrix is sparsely filled with measurements,missing data is interpolated, the inverse is taken, and then heuristicsare applied to determine the real dimension, or number of channels andstimulation groups, and the degree of simultaneity possible.)

[0073] Stimulation groups are then coded into a pulse table, similar tothat described in U.S. Pat. No. 6,002,966, or U.S. patent applicationSer. No. 09/322,712, previously referenced. Contacts within stimulationgroups are then fit to the patient's dynamic range using objective andbehavioral means for a given or optimum pulse-width. From thesemeasures, a target compliance voltage is derived based upon what isexpected to be the optimum stimulation rate. (The narrower the pulsewidths, the higher the rate, and the higher the compliance voltage.) Inorder to minimize power dissipation, it is preferred that all contactsbe set as close a possible to the target compliance voltage for theirrespective maximum stimulation levels. This is typically accomplished byvarying pulse width. Noting that high stimulation rates and manychannels may be necessary, use of the stimulation groups may benecessary. To simplify the system and the interactions, pulse widths ofall the contacts within a stimulation group are kept the same. Thiscomplicates the software somewhat because it is the software thatchanges the pulse width and amplitude to reach the target compliancevoltage, while the clinician programming the device is able to onlyincrease (or decrease) the amplitude.

[0074] The software development and fitting speed are simplified bymaking modifications to the pulse table that has been used in the past,e.g., as disclosed in the '712 patent application. Further details ofthe state machine are found in the '712 patent application with thefollowing simplifications and changes.

[0075] 1. The Data path is simplified because the pulse table has directaccess to the filter banks in an unconstrained fashion, the currentsource has a limited number of configurations; there are no multipliersor shifters with the exception of a possible DAC current range in thedigital implementation.

[0076] 2. The primary complicating factor for software in the systemshown in the '712 patent is a uniform pulse width. This is eliminated bythe addition of a pulse width entry type associated with each hold bit.This entry significantly reduces memory in strategies with non-uniformpulse widths.

[0077] 3. An increased memory (2× or 4×) size allows the pulse tablegeneration software to not require extensive optimization.

[0078] 4. The use of two word commands is permitted across frameboundaries.

[0079] The Pulse Table (included within the MCU 4) drives the PulseGenerator. The Pulse Generator is responsible for selecting the correctAcoustic Processing Bank taps, configuring the Current Source and thensending a code to the Active Electrodes.

[0080] Codes to the Active Electrodes are of special concern becausethey require that each contact within a bank have a uniqueidentification (ID) code. For a 16-contact system, this requires 4 ID's;for a 64-contact system, this requires 16 ID's. Code is implemented inany of three ways: electrically fusible links, laser fusible links, orhard-wire connection performed during IC Mask operation. Chargedeposition links are not used because of possible moisture issues.

[0081] The code consists of an identification pre-amble which includes2-bit positions for each contact in the bank (e.g., 0001 0000 0010 00000001 0000 0000 0000 indicating the 6^(th) Medial, 13^(th) Medial and13^(th) Lateral contact are connected) and a post-amble with a doubleparity and a unique end code (AB₁₆). The clock is embedded in the codesusing Bi-phase or Manchester encoding so that a local oscillator is notneeded. If the contact does not receive a valid code, or if its bitposition is not set, it is turned off and reset. Within a bank, manycontacts may be turned on simultaneously using the same driving signal,thereby effectively forming larger contacts or adjustable area contactsto adapt to the particular patterns of damage or pathology in thepatient's cochlea or other tissue. Using the indifferent electrode withconnections to various Lateral Contacts can also create patterns ofactivation.

[0082] One feature provided by the present invention is simultaneousN-of-M strategy. The idea underlying non-simultaneous N-of-M strategy isthat if an implant device has M physical stimulation sites, only Nrelevant sites need to be stimulated during each repetition of thestimulus pattern. This has the effect of enabling higher repetitionrates and potentially transferring more relevant information to thepatient. Enhancing N-of-M with simultaneous stimulation has thepotential to transfer even more information to the patient.

[0083] In order to make simultaneous N-of-M strategies viable in apractical system, the following issues should be considered:

[0084] 1. Contacts or stimulation sites must be grouped into staticallyassigned or dynamically changing stimulation groups. The presentinvention preferably uses statically assigned groups to reduce thesystem complexity and power.

[0085] 2. Any suitable criteria may be used for deciding when astimulation group should be skipped or temporally re-sequenced. Onetime-dependent technique that may be used for this purpose is to look atthe sound energy (made by using a weighted sum of the logarithms of thesound envelope energy for each stimulation group) in all of thestimulation groups simultaneously, sort the weighted sums for each groupand send the top N contributors. This technique of sorting is veryexpensive in terms of memory and circuitry and power. A second techniqueis for the system to have an adaptive threshold where N stimulationgroups are attempted but will vary from strategy frame to strategyframe. This method is implemented by means of a control loop varying athreshold that uses the energy estimate of each stimulation group asthey come up for possible stimulation. Finally, a scheme thatincorporates the pyscho-physical masking parameters as a criteria canalso be used where the measured transfer function of the neuralinterface is used to judge the relevance of the stimulation group to thepatient. This last method requires models of the neural interfacetemporal integrators for at least each Stimulation Group.

[0086] 3. Once Stimulation Groups are determined, pulse widths withinthe group should be kept identical so as not to change the interactions.Therefore, an additional constraint allows pulse width changes betweengroups, but not within groups. Intergroup pulse width changes aredesigned to minimize strategy frame periods while maintaining a constantmaximum compliance voltage to minimize power dissipation.

[0087] The current sources 10 and 21 used by the present invention arespecialized to always charge balance each current pulse delivered on asingle contact or a set of contacts. Contact configurations cannot bechanged until charge balance is achieved. Charge balance is achieved byfirst calibrating and then compensating for the bipolar offset of thecurrent source. Calibration of the system is performed by issuing a stepfunction command on the electrode and using the back telemetry system tomonitor its decay to thereby determine its transfer function. Once thetransfer function is known, the current source is driven to servo thecurrent to achieve a net zero charge balance before allowing the systemto proceed, thereby creating a hardware interlock feature. Such hardwareinterlock feature presents the system from proceeding until chargebalance is achieved. Since, in the preferred embodiment, up to fourcurrent sources can be operating simultaneously, all four current sinksmust be balanced before proceeding.

[0088] As mentioned earlier, stimuli preferably occur in StimulationGroups 56, as shown in FIG. 7 (which illustrates five stimulationgroups). Within a Stimulation Group 56, up to four independent currentsources, shown, e.g., in FIG. 1, can stimulate selected non-overlappingcontacts. After stimulation, there is a forced or reserved recoveryperiod 57 which must elapse before stimulation of the next group. (Note,that for the example shown in FIG. 7, the forced or reserved recoveryperiod 57 between group #3 and group#4 is zero, which the other forcedor reserved recovery periods have a finite width.) Each group iscontrolled by a Pulse Generator 13 that first resets an Integrator inthe group and disconnects any medial contacts 19A and optionally lateralcontacts 20A. Then, the pulse generator 13 selects the appropriate tapthrough the cross-point switch 7 (FIG. 1) and connects the selectedmedial and lateral contacts. The amplitude is determined by the tapwhere the value is held.

[0089] Current balancing is best realized using a current balancingsystem as is illustrated in FIG. 6, and with waveforms as depicted inFIG. 6A. It is noted that the particular hardware circuitry shown inFIG. 6 represents a particular configuration of the circuitry shown inFIG. 1. That is, the circuitry shown in FIG. 6 illustrates particularelements and components of the circuitry shown in FIG. 1, configured toprovide the current balancing function. As seen in FIG. 6, the currentbalancing system consists of a long-term adjustment found during systemcalibration and a control loop. The control loop comprises acompensation network 115 that is adapted to the transfer function, ahighly accurate integrator 152 to estimate charge, a zero detector 154to signal charge balance, and a limiter 153 to limit the amount ofcurrent that can be driven onto the electrode to balance charge. A PulseTable included within the MCU 4 normally keeps the limiter 153 set tozero with a MaxComp line 161 set to zero until the Pulse Table expectsbalanced charge. When balanced charge is expected, the MaxComp line 161is raised to the maximum level of stimulation needed to achievebalancing.

[0090] Analog circuitry used within the prosthesis is programmed via theMCU using DACs that set various parameters. Digital Banks areimplemented either by semi-custom logic or by signal processingperformed within the MCU/DSP 4 itself. Digital Banks require theaddition of a 14-bit effective ADC (in place of 2). In the event theAcoustic Processing Bank is digital, the current source becomes acurrent mode digital-to-analog converter (DAC). For the analog bank, thecurrent source is an amplifier.

[0091] As described above, it is seen that additional complexity hasbeen included in the prosthesis system in order to allow more thansixteen channels and in order to have a connector disconnect. Asimplified version of the system can be made by moving the activeelectrode electronics into the hermetic space of the sealed housing 100(FIG. 1), and then providing an hermetic feed-through for each electrodecontact. Additional simplification may be realized by adding a capacitorin series with each contact, thereby eliminating the need for currentbalancing electronics.

[0092] While the invention herein disclosed has been described by meansof specific embodiments and applications thereof, numerous modificationsand variations could be made thereto by those skilled in the art withoutdeparting from the scope of the invention set forth in the claims.

What is claimed is:
 1. An active electrode array for use with animplantable neural stimulator, wherein the active electrode arraycomprises at least four banks of active electrodes, wherein each bank ofactive electrodes includes a plurality of active electrodes, whereineach active electrode comprises a plurality of individual electrodecontacts and an active switch integrally formed with the individualelectrode contacts for individually activating the plurality ofindividual electrode contacts with a selected electrode stimulationcurrent in response to electrode control signals.
 2. The activeelectrode array as set forth in claim 1 wherein the plurality ofindividual electrodes included within each active electrode comprises atleast one lateral electrode contact and at least one medial electrodecontact. 3 The active electrode array as set forth in claim 2 whereineach active electrode includes a silicon die and switching circuitryhermetically sealed on the silicon die and operatively connected to thelateral and medial electrode contacts, wherein the switching circuitryresponds to the electrode control signals to selectively activate one orboth of the medial or lateral electrode contacts. 4 The active electrodearray as set forth in claim 3 wherein each bank of the active electrodescomprises a stack of the silicon dies of each active electrode belongingto that bank, over-molded with silastic.
 5. An active electrode arrayadapted for use with an implantable tissue stimulating prosthesis,wherein the active electrode array comprises: a plurality of activeelectrodes; wherein each active electrode includes switching circuitrybuilt into the electrode array and a plurality of individual electrodecontacts that may be individually activated by electrode control signalsapplied to the switching circuitry.
 6. The active electrode array ofclaim 5 wherein the active electrode array includes at least four activeelectrodes.
 7. The active electrode array of claim 6 wherein theplurality of individual electrode contacts included within each activeelectrode comprises at least one lateral electrode contact and at leastone medial electrode contact.
 8. The active electrode array of claim 7wherein the switching circuitry of each active electrode comprises:decoding circuitry, a first switch coupled to the decoding circuitry andthe at least one lateral electrode contact, and a second switch coupledto the decoding circuitry and the at least one medial electrode contact,wherein the decoding circuitry responds to the electrode control signalsand causes the first and second switches to selectively activate one orboth of the medial or lateral electrode contacts.
 9. The activeelectrode array of claim 8 wherein the decoding circuitry and first andsecond switches of each active electrode are formed on a substrate die,and wherein the medial and lateral electrode contacts of the activeelectrode are formed on opposing edges of the substrate die, and whereinat least four of said substrate dies are stacked and over-molded withsilastic to form the active electrode array.
 10. The active electrodearray of claim 9 wherein the active electrode array comprises aplurality of active electrode banks, wherein each active electrode bankincludes a plurality of active electrodes.
 11. The active electrodearray of claim 10 wherein at least one active electrode in each activeelectrode bank includes a built-in strain gauge, wherein the straingauge is adapted to measure stress across the substrate die.
 12. Anactive electrode array comprising a flexible carrier in which n+1 wiresare embedded, where n is an integer of at least 4; an active electrodearray at or near a distal end of the flexible carrier comprising atleast 2n electrode contacts; switching circuitry located at or near thedistal end of the flexible carrier adjacent the electrode contacts, saidswitching circuitry being responsive to control signals presented on atleast a plurality of the n+1 wires so as to direct a stimulation signalpresented on another of the n+1 wires to a selected pair of theelectrode contacts; and means for connecting the n+1 wires at a proximalend of the flexible carrier to electronic circuitry adapted to generatethe control signals and stimulation signal; whereby at least 2nelectrodes located at the distal end of the flexible carrier may beconnected for individual control through no more than n+1 wires embeddedwithin the flexible carrier.
 13. The active electrode array as set forthin claim 12 wherein the at least 2n electrode contacts included withinthe active electrode array comprises at least n lateral electrodecontacts and at least n medial electrode contacts.
 14. The activeelectrode array as set forth in claim 13 wherein the switching circuitrycomprises a silicon die on which switching circuitry has been formed andhermetically sealed and operatively connected to the lateral and medialelectrode contacts, wherein the switching circuitry responds to theelectrode control signals to selectively activate selected ones of themedial or lateral electrode contacts.
 15. The active electrode array ofclaim 12 wherein the switching circuitry of each active electrodecomprises: decoding circuitry, a first switch coupled to the decodingcircuitry and the lateral electrode contact, and a second switch coupledto the decoding circuitry and the medial electrode contact, wherein thedecoding circuitry responds to the electrode control signals and causesthe first and second switches to selectively activate one or both of themedial or lateral electrode contacts.
 16. The active electrode array ofclaim 15 wherein the decoding circuitry and first and second switches ofeach active electrode are formed on a substrate die, and wherein themedial and lateral electrode contacts of the active electrode are formedon opposing edges of the substrate die, and wherein at least four ofsaid substrate dies are stacked and over-molded with silastic to formthe active electrode array.